The statements in this section merely provide background information related to the present disclosure. Accordingly, such statements are not intended to constitute an admission of prior art.
Vehicle powertrain systems include one or more torque actuators coupled to a transmission that transfers torque to a driveline for tractive effort. Known torque actuators include internal combustion engines and electric machines. A known hybrid powertrain system employing an electric machine includes an electric machine coupled to a crankshaft of the engine to function as a torque actuator in place of an alternator. The electric machine can regenerate electrical energy for storage in an electrical energy storage device when the engine is in operation and can function as a starter for the engine. Another known powertrain system employing an electric machine includes an internal combustion engine coupled to a first axle of the vehicle and an electric machine coupled to a second axle of the vehicle. The electric machine can provide tractive torque to the second axle of the vehicle for enabling an electric mode of vehicle operation.
Meshed driveline components, e.g., transmission splines, a chain, a final drive, a differential and/or interleafed gear teeth on planetary gear sets, have clearances that are a result of manufacturing tolerances and component design specifications. Gear lash, i.e., play or slack in relative rotational positions of the meshed driveline components, results from the clearances between the meshed driveline components. When a vehicle is operating in an electric mode by an electric machine providing tractive torque to a rear axle of a vehicle, the front axle coupled to the engine may drag the vehicle transmission so that gear lash contact is present on a “braking” side of the meshed driveline components. When an engine is started and a desired gear ratio of the transmission is selected, gear lash contact changes onto a “driving” side of the meshed driveline components resulting in undesirable driveline clunk. It is desirable to eliminate undesirable driveline clunk when a hybrid powertrain transitions from an electric operating mode to an operating mode requiring the engine to be started for providing tractive torque to the front axle of the vehicle.